A power plant, using a low boiling point medium, for recovering heat energy from a low-temperature heat source which has not been utilized in conventional geothermal power generation using a steam turbine and for generating a power has attracted special attention as an energy recovery device recently (see Patent Literature 1).
FIG. 7 shows a basic system diagram of a conventional power plant using a low boiling point medium. This power plant exchanges heat between a medium having a lower boiling point than water and a heat source by a vaporizer 100 to evaporate this medium, rotates a turbine 101 by this medium vapor, and operates an electric generator 102 by the rotational force, thereby obtaining a power. The medium exiting from the turbine is condensed by a condenser 103 and is delivered back to the vaporizer 100 via a preheater 105 by a circulation pump 104. Then, the above cycle is repeated.
In general, when a medium with a high vapor pressure (i.e., a low boiling point) is used, vaporization by the vaporizer is easy but condensation by the condenser is difficult. To the contrary, when a medium with a low vapor pressure (i.e., a high boiling point) is used, vaporization is difficult but condensation is easy. From this point of view, a medium which maximizes an enthalpy difference (heat difference) between a turbine inlet and a turbine outlet is selected as a medium to be used. For example, n-pentane (nC5H12) is mainly used as a natural medium used in a condition where a temperature of a geothermal heat source is from 130° C. to 140° C. and a temperature of a cooling source is from 15° C. to 30° C.
The cooling source of the condenser is generally circulating cooling water or an atmosphere. Therefore, the temperature of the cooling source is largely different between winter and summer. Thus, in a case where the condenser is designed only based on a cooling performance required in summer, the cooling performance of the condenser is further enhanced when the temperature of the cooling source drops in winter.
As shown in FIG. 4, however, the vapor pressure of n-pentane falls to 101 kPa or lower when its temperature falls to 36° C. or lower. Therefore when the temperature of the outlet of the condenser drops to 36° C. or lower in winter, a medium flow path may be the atmospheric pressure or lower. In this case, it is likely that an air intrudes into the medium flow path from the main body of the condenser and various joints of a connection pipe of the condenser or a mechanically sealed portion of the turbine shaft, for example.
Thus, as a device for removing the air intruding the medium in a plant related to power generation, Patent Literatures 2 to 6 described below are known.
Patent Literature 2 discloses a binary power plant using water instead of a low boiling point medium, equipped with an air extraction device for extracting an air from drain water of a condenser.
Patent Literature 3 discloses a power system including a power cycle circuit 10 which circulates a working fluid in which a high boiling point medium and a low boiling point medium are mixed through a vapor generator 1 for heating a solution of the working fluid and generating a vapor, a steam turbine 2 which is driven by the vapor supplied by the vapor generator 1, a condenser 3 for cooling the vapor released from the steam turbine to condense it to the solution, and a feed pump 16 for supplying the solution supplied from the condenser 3 to the vapor generator 1, in that order, wherein a concentration of the low boiling point medium of the working fluid in the condenser 3 is determined to provide a pressure around the atmospheric pressure as the lowest pressure which can be generated in the condenser 3 in the power cycle circuit 10.
Patent Literature 4 discloses a plant which includes a chamber having a piston therein provided above an upper portion of a condenser, a valve connecting a space below the piston in the chamber to the condenser, a cooling means cooling a lower portion of the chamber by a coolant through a wall, and a discharge valve connected to the lower portion of the chamber.
Patent Literatures 5 and 6 disclose a plant including: a tightly sealed chamber above an upper portion of a condenser, the chamber being provided with a movable diaphragm for dividing the inside of the chamber into an upper portion and a lower portion; two flow rate control valves arranged between the condenser and the lower portion of the chamber in series; a cooling means for cooling the lower portion of the chamber with a coolant through a wall; and a discharge valve connected to the lower portion of the chamber.